The invention relates to a sigma-delta modulator, comprising: an integrator, a quantizer connected to an output of the integrator, a feedback circuit comprising a D/A-converter having an input connected to an output of the quantizer, and an adder having an output connected to an input of the integrator and having a first input which forms the input of the sigma-delta modulator and having a second input connected to an output of the feedback circuit.
Such sigma-delta modulators are generally known but particularly from the article: xe2x80x9cA 0.2 mW CMOS xcexa3xcex94Modulator for Speech Coding with 80 dB Dynamic Range.xe2x80x9d, IEEE Journal of Solid-State Circuits, vol. 31,No.12, December 1996.
Due to the increasing use of these circuits in a wide field of applications, a need has developed for circuits which are flexible in terms of input level, gain, process parameters, etc. From the document above it is known to vary the input level by switching of resistors in the integrator, which leads to a certain degree of flexibility in input level. However, there is also a need for flexibility in more parameters.
Therefore, the invention provides such a sigma-delta modulator, wherein the feedback circuit has an adjustable feedback factor and wherein the sigma-delta modulator includes an adjusting member for adjusting the feedback factor of the feedback circuit. These features provide adjustability of the feedback factor, which leads to a substantially better adaptability to different applications.
According to a first preferred embodiment the feedback circuit includes an attenuator which is adjustable by the adjusting member. This feature provides in a easy implement and easy to understand configuration.
According to a second preferred embodiment the adjustable attenuator concludes switched capacitors. Capacitors can be implemented more easily in an integrated circuit than resistors, so that this structure leads to less surface area on the chip, which results in lower costs.
Another embodiment has the features that the feedback circuit, the adder and the integrator are combined to a united circuit and the united circuit includes at least one adjustable current source connected to the common contact of a switch having two other contacts connected each to one terminal of a capacitor which performs the integrating function of the integrator, the switch being controlled by the D/A-converter and the adjusting member being operable to adjust the magnitude of the current delivered by said current source.
The use of a united circuit with a switch and a current source is known in this field. It provides an attractive solution for all these functions with a limited number of components. This embodiment combines these advantages with the advantages of the invention. This combination is only possible by the use of adjustable current sources, which can be realised easily.
Although other solutions, such as gradually controllable current sources are possible, an exceptionally easy solution is obtained when the current source is composed of a number of partial current sources each connected to the common contact of the switch through respective further switches which are controllable by the adjusting member. Even with a limited number of switches a wide range of currents can be controlled, which leads to a wide control range. This range can even be enlarged or refined when the current sources have mutually different current values. It is, however, also possible to use this feature to limit the number of current sources and switches.
Stability of sigma-delta modulators can be a problem in modulators with high closed loop gains. This may also be the case with the present invention when the feedback loop factor is increased. This increase may lead to such a high closed loop gain that the system becomes marginally stable or unstable.
To avoid such an undesired situation, a preferred embodiment is characterized in that the gain of the combination of the integrator and the quantizer is adjustable. These features provide an extra possibility of controlling the overall closed-loop gain so as to avoid instability.
Furthermore, control can be simplified when, as proposed in another preferred embodiment, the gain of the combination of the integrator and the quantizer is adjustable by the control member. The control of the gain of the feedback loop can then be related to the control of the gain of the rest of the closed loop. A particular adjustment strategy can then be used to avoid potentially instable situations.
An example of such an adjustment strategy is that the adjustment member is operable to control the gain of the combination of the integrator and the quantizer in such a manner that the product of their individual gain factors is constant. This strategy automatically ensures stability if the product is chosen small enough.
The preceding embodiments all relate to the adjustability of the gain factors, i.e. the situation in which the adjustments are made and are fixed during the production of the integrated circuit containing the components of the sigma-delta converter, or shortly thereafter during an initialization procedure.
The invention is, however, not limited to such a situation; it is very well possible to adjust the gain also during actual use of the circuits. In such a situation the word xe2x80x98controlxe2x80x99 is used instead of the word xe2x80x98adjustmentxe2x80x99.
Therefore, in a preferred embodiment the adjusting member is adapted to continuously control the feedback factor of the feedback loop. This feature relates to the situation in which only the gain of the feedback loop is controlled. However, this feature is also applicable in a situation in which the overall closed-loop gain is also controlled. The feature that the adjusting member is adapted to continuously control the gain of the combination of the integrator and the quantizer can then be used.